Hey so I just got a successful downgrade on N3DS (according to the source, has 100% success rate on N3DS and O3DS with 7/7 flawless successes once sysUpdate worked). Just freshly reformat your SD card as FAT32 with a 32MB cache (I used SmartDisk FAT32 Tool, it did it automatically). Put HBL, the .CIAs from that ISO site, and the sysUpdater from that site too. I used browserhax as my initial entrypoint and then installed menuhax. sysUpdater worked on my second try with menuhax (try launching FTPony once before you launch sysUpdate!) and installed flawlessly with no errors.
Hope this helps.
[SOURCE] (reddit; u/NoYouMayNotAMA)
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@TheStoneBanana Could you maybe pin this? The visibility could help someone! Just don't forget to credit OP)